In current integrated circuits (ICs), such as complementary metal-oxide-semiconductor radio-frequency (CMOS RF) circuits, inductor is an important electrical device. Performance parameters of an inductor can directly affect performance of an IC. Conventionally, most inductors in ICs are planar inductors, such as planar spiral inductors. The planar inductors are formed by winding a metal wire on a surface of a substrate or a dielectric layer. In comparison with traditional coils or coil inductors, the planar inductors have advantages such as low cost, ease of integration, low noise and low power consumption. More importantly, the planar inductors can be compatible with current IC processes.
Performance of an inductor is characterized by a quality factor Q of the inductor. The quality factor Q is a ratio of energy stored in the inductor to energy loss during each oscillation cycle. The higher the quality factor Q of the inductor, the greater the efficiency and the better the performance of the inductor. The energy loss of the inductor is an important factor that affects the value of Q. Substrate loss in an IC accounts for the largest proportion of the energy loss of the inductor. That is, the substrate loss is an important factor that affects the quality factor Q of the inductor.
Conventionally, on one hand, a planar inductor is disposed on an IC, so electric field lines can enter a substrate and lead to electric charge movements in the substrate to generate a coupling substrate current. The current can lead to ohmic loss in the substrate. On the other hand, an alternating current in the inductor generates an alternating magnetic field (i.e., an induced alternating magnetic field), which can vertically enter the substrate through the surface of the substrate to generate an alternating eddy current in the substrate. The eddy current can dissipate electric energy (converted from magnetic energy) as Joule heat, which causes eddy current loss. The ohmic loss and the eddy current loss can result in a significant substrate loss, which can significantly reduce the quality factor Q.
Therefore, currently, a ground shield structure is provided between the inductor and the substrate. The ground shield structure serves to shield the electric field lines and the induced magnetic field lines of the inductor, such that the electric field lines and the induced magnetic field lines can terminate at the ground shield structure instead of entering the substrate. The substrate loss can thus be reduced.
Although the existing ground shield structure can reduce the substrate loss, in practical applications, after the existing ground shield structure is introduced into a semiconductor device containing the inductor, the quality factor Q of the inductor is not improved. To the contrary, the quality factor Q of the inductor is even reduced in some bands of operating frequency of the inductor.
Referring to FIG. 1, a curve 01 corresponds to a Q value of an inductor not having a ground shield structure, and a curve 02 corresponds to a Q value of an inductor having an existing ground shield structure. In an operating frequency band of the inductor that is higher than 10×109 Hz, the Q value of the inductor having the ground shield structure is not significantly greater than the Q value of the inductor not having the ground shield structure. Moreover, in an operating frequency band of the inductor that is lower than 10×109 Hz, the Q value of the inductor having the ground shield structure is lower than the Q value of the inductor not having the ground shield structure.